Question 230·Medium·Boundaries
The research team concluded that the lake water, though once considered pristine, now harbors elevated levels of _____ and mercury. These heightened concentrations, they argue, demand immediate remediation.
Which choice completes the text so that it conforms to the conventions of Standard English?
For boundaries questions, first decide what structure you’re in: (1) two independent clauses, (2) a dependent clause/nonessential phrase, or (3) a list. If you see a straightforward series of items (especially with “and” before the last item), use commas to separate the items; reserve semicolons for joining independent clauses or separating complex list items that already contain commas, and use a colon only to introduce a list after a complete clause.
Hints
Look for a series
The words “and mercury” suggest you’re in the middle of a list. How many items are being named?
Recall standard list punctuation
In a simple series, items are separated by commas: item 1, item 2, and item 3.
Be cautious with semicolons and colons
Semicolons usually separate independent clauses (or complex list items with internal commas), and colons typically introduce a list—neither belongs between two simple list items.
Step-by-step Explanation
Identify what comes after the blank
After the blank, the sentence continues with “and mercury,” which signals that the sentence is listing items in a series.
Determine the punctuation needed in a series
In a simple list of three items, items are separated by commas:
- item 1, item 2, and item 3
Here, the items are the pollutants (phosphorus, nitrates, and mercury).
Eliminate punctuation that does not separate list items
A semicolon, dash, or colon is not used between items in a simple series like this. Those marks would incorrectly interrupt the list structure.
Choose the option that correctly punctuates the series
The correct choice uses a comma between the first two items: phosphorus, nitrates,.